This use to mean something that is very easy to recall, or something that once learned is quite hard to forget, but as of recent times this is no longer accurate. From now on, in honor of Sleepy Joe Biden, it will be used to refer to a complete fail or an embarrassing mistake, or when somebody forgets something very basic that they should know or something they knew how to do for a long time.
1.
Joe Biden's presidency is just like riding a bike.
2.
I can't believe you forgot the words to that song that you knew by heart all of your life, that was just like riding a bike! Come on man!